A major unresolved issue in developmental neurobiology is how growing axons navigate toward their target areas. This proposal is directed to identify the mechanisms that guide growing axons in vivo. Determination of these mechanisms will provide the necessary information for continued research into the cellular, molecular, and genetic bases for axonal guidance, as well as insight into possible mechanisms for nerve regeneration. The sympathetic nervous system of the chick has proven to be extremely useful for axonal guidance studies in vivo. Its anatomy is relatively simple, the development well understood, and the presumptive pathways of growing axons are predictable. Previous studies from this laboratory have shown that sympathetic preganglionic projections are segment specific, correct from the outset, and independent of the target tissue. More recent results have shown that the spatiotemporal pattern of preganglionic axon outgrowth can be influenced by the surrounding somitic mesoderm. Taken together, these findings suggest that some properties of the neuron/neural tube, or the adjacent somitic mesoderm, can influence the specificity of preganglionic projections. In order to determine which mechanisms are involved in the guidance of sympathetic preganglionic axons, I have proposed four independent models. Each model is designed to test only one potential influence on sympathetic preganglionic outgrowth, and the models are not mutually exclusive. The models are based on the following potential influences: l) the intrinsic identity of the somitic mesoderm; 2) the maturation of the somitic mesoderm; 3) the intrinsic identity of the preganglionic neurons/neural tube segments; and 4) interactions with neighboring preganglionic neurons. Because of the expertise that I have gained in experimental manipulations of the chick embryo, it is possible to test each of these models in vivo. The experimental technique involves mainly heterochronic and/or heterotopic neural tube or somite transplantations, followed by evaluation of changes in preganglionic projections. This technique has yielded some very fruitful results in the past. For each model, multiple experiments are proposed in order to increase the reliability of the results. The experiments are designed to alter only one variable, such that positive results lend themselves to only one interpretation--that the model is supported. Preliminary studies have provided evidence in support of some of the proposed models.